Dr. McMahon's long term objective is to study the role played by cell signalling in the regulation of growth and patterning in the mammalian CNS. He demonstrated that the normal role of the signalling molecule encoded by the mammary proto-oncogene Wnt-1 is to regulate brain development. He seeks to investigate a number of issues which have arisen from examination of Wnt-1-/- mice. At the cellular level it is not clear how the observed anatomical deletion of the midbrain and anterior hind brain is brought about. For example, Wnt-1 may act as a regional determinant, loss of Wnt-1 resulting in cell fate changes. This will be tested by direct cell marking studies in whole embryo culture and by following the fate of cells expressing a histochemically detectable reporter construct driven by a Wnt-1 enhancer. Alternatively, if Wnt-1 acts as a trophic stimulus, cessation of mitosis and/or cell death may generate the observed phenotype. Mitotic activity and cell death will be examined in mutant embryos. Uncoupling of the normal spatial distribution of Wnt-1 in the CNS may generate additional phenotypes which reflect the normal role of the Wnt-1 signal. This is achieved by ectopic expression of Wnt-1 in transgenic mice under the control of a neural specific regulatory element. Caudal areas of the CNS express Wnt-1 and Wnt-3a and may have redundant signalling functions. This issue will be examined in single (Wnt-3a) and double (Wnt-1 and Wnt-3a) mutant embryos, and by attempting to rescue Wnt-1 mutant embryos by ectopic EN-1 expression.